Method and apparatus for manufacturing tyres for vehicle wheels

ABSTRACT

In a method for manufacturing tyres for vehicle wheels, a green tyre, once formed, is transferred into a vulcanisation mould and pressed against the holding walls of a moulding cavity. Concurrently with the pressing step, a fluid present between the green tyre and the holding walls is evacuated through vent valves. The tyre is brought into contact with the closing head of each valve so as to push it toward a closed position of the valve itself. The closing head has a distal portion facing the moulding cavity and includes at least one grip element for the tyre blend, so that the closure member is pulled toward the open position while the moulded and cured tyre is moved away from the inner surface of said moulding cavity.

The present invention relates to a method and an apparatus formanufacturing tyres for vehicle wheels.

Generally, in tyre manufacture for vehicles wheels, subsequently to astep of building a green tyre through assembling of respectivecomponents of elastomeric material some of which are provided withsuitable reinforcing structures, a moulding and curing treatment isprovided to be carried out to the aim of causing structuralstabilisation of the tyre through cross-linking of the elastomericcomponents of same and, as generally required, of impressing a desiredtread pattern into the tyre itself, as well as possible distinctivegraphic marks at the tyre sidewalls.

To this aim, the green tyre is introduced into a suitably heatedvulcanisation mould, having a moulding cavity conforming in shape to thefinal conformation to be given to the tyre itself. After carrying outclosure of the mould, the green tyre is pressed against the holdingwalls of the moulding cavity and the heat required for tyrevulcanisation is simultaneously transmitted thereto. To reach thispurpose, for example, expansion of a bladder of toroidal conformation iscaused within the tyre through admission of steam under pressure intosaid bladder, so as to bring the bladder into contact with the innersurface of the tyre and compress the latter against the holding walls ofthe moulding cavity.

When vulcanisation has been completed, the mould is opened to enableremoval of the tyre and arrange the mould for a newmoulding/vulcanisation cycle.

Once the green tyre has been introduced into the mould, in order toprevent air pockets or pockets of other fluid possibly used in thevulcanisation process from being entrapped between the holding walls ofthe moulding cavity and the outer surface of the green tyre, the mouldsof the conventional type are provided with a plurality of venting ductspassing through the mould wall and opening into the moulding cavity,said ducts lying at right angles to the holding walls of said mouldingcavity. These ducts during the step of pressing the tyre against theholding walls of the moulding cavity, enable evacuation of possible airpockets and ensure a perfect adhesion of the outer surface of the greentyre to the holding walls of the moulding cavity. Adoption of ductspassing through and opening into the moulding cavity however, involves aplurality of problems due to seeping of the green-tyre blend into theducts themselves during the moulding step. In fact, the blend entrappedin the ducts is subjected to vulcanisation and forms a plurality ofthreadlike outgrowths on the finished tyre that are known in thespecific technical field as “flashes” or “burrs” and are generallyremoved in a work station dedicated to this purpose, when the tyreproduction process has been completed. Alternatively, these threadlikeoutgrowths are not removed from the outer surface of the finished tyreand in this case they involve worsening of the aesthetic-qualitativelevel of said tyre.

The state of the art proposes many solutions aiming at reducingformation of these threadlike outgrowths.

For example, in document CA-2,190,720 use of a movable valve is providedthat is inserted in each of the holes formed in the mould walls, saidvalve comprising a stem and a closing head associated with said stem.The closing head has a frustoconical shape provided with a flat surfacefacing the inside of the moulding cavity. A return spring pushes thevalve to the open position at which the closing head lies spaced apartfrom a respective seat of a mating shape, designed to receive said stemand closing head. The valve is forced to the closed position by virtueof the thrust exerted thereon by the tyre blend during the moulding stepand comes back to the open position when the tyre is removed from themould.

Document GB-2,339,163 discloses a venting device comprising a closingelement slidably disposed within a main body of said device. A first anda second springs enable movement of the closing element—relative to themain body—so as to allow opening and closing of the device. At the innersurface of the moulding cavity, the main body of the device is providedwith a cone-shaped seat. At the upper end of the closing element thereis a closing surface of conical shape, matching the shape of thecone-shaped seat of the main body of the device. Closure of the valvetakes place when the cone-shaped seat and closing surface are engagedwith each other through movement of the closing element within the mainbody.

Document EP-1,380,397 discloses a venting apparatus positioned in a ventopening of a mould. Said apparatus comprises: a venting duct; a valvebody inserted in said duct; a spring surrounding a lower portion of thevalve body to supply said valve body with an elastic force; and aretainer in engagement with a lower end portion of the valve body tocontrol vertical displacement of the valve itself. The inner diameter ofthe venting duct gradually increases towards the moulding cavity todefine a tapered shape. In addition, the valve body has a frustoconicalclosing head provided with an inclined lower surface the inclination ofwhich is different from the inclination of the inner wall of the ventingduct. In this manner, when the closing head comes into contact with theinner surface of the duct, contact does not take place along a surfacebut along a circumference because in radial section the contact betweenthe lower surface of the closing head and the inner surface of the ductis of the punctual type.

The Applicant has ascertained that, following the vulcanisation process,the blend seeping into the evacuation duct of the vent valve, tends tosettle at least partly in the contact region between the closing head ofthe valve and the respective valve seat (i.e. the inner wall of theduct). This situation is particularly disadvantageous because, due tothe presence of this blend, the closing head and the wall of the ducttend to glue together until, after repeated cycles, a definitivestoppage of the valve occurs and, as a result, said valve needs to bereplaced in order to restore a correct evacuation of the air from themoulding cavity.

The Applicant has therefore perceived the necessity to increase thethrust required for carrying out opening of the valve at the end of eachvulcanisation cycle, i.e. when the finished tyre (moulded and cured) isextracted from the mould, in order to avoid or at least reduce the riskof stoppage of the valve itself.

The Applicant has further ascertained that, in the open position, thelower tapered portion of the closing head of the known art valves (theabove described valves, for example) faces the inside of the mouldingcavity and greatly projects relative to the inner surface of saidcavity, this surface being defined by the holding walls of said mouldingcavity. This aspect is particularly disadvantageous because in someregions of the mould, in particular at the mould regions designed toform the tyre shoulders, the blend tends to flow tangential to the mouldwall and to approach the valve head in a lateral direction relative tosaid valves. In this way the blend exerts a side thrust on the lowertapered portion of the closing head, this thrust generating a forcedirected to move the closing head away from the valve seat and to openthe valve itself instead of closing it. Therefore, this forcecounteracts the thrust exerted by the blend approaching the closing headin a direction perpendicular to the inner wall of the moulding cavityand aimed at carrying out the valve closure.

The Applicant has further noticed that the presence of blend interposedbetween the closing head and the seat of the valve compromises a correctclosure of the valve itself and brings to formation of faults on thefinished product. Repetition of this phenomenon during subsequentvulcanisation cycles further involves an increasingly greater storage ofcured blend within the valve, so that, cycle after cycle, an unavoidablestoppage of the valve itself will be caused.

Finally, the Applicant has ascertained that the side thrust of the blendon the closing head of the valve can cause misalignment of the valvestem relative to the duct in which the valve slides. Since a passageport necessary to enable the air to flow out of the moulding cavity isdefined between the stem and duct, the amount of the steminclination—due to the above described side force—can be of such a valuethat sliding of the valve within the valve seat and a correct air floware impaired.

The Applicant has therefore perceived the necessity to provide the mouldwith vent valves capable of preventing, or at least greatly reducing,blend seepage into the ducts of said valves and therefore avoid stoppageof same, as well as formation of the above mentioned threadlikeoutgrowths generally found in the known art embodiments.

The Applicant has found that the above mentioned problems can beeliminated or at least greatly reduced by arranging on the closing headof the valve, a region of engagement with the tyre blend in such amanner that, when the finished tyre is moved away from the inner surfaceof the mould, the tyre itself co-operates in this operation by exertinga pulling action on the closing head of the valve towards the closedposition of the mould. In this way, opening of the mould helps incarrying out the possible separation of the closing head of the valvefrom the respective seat.

The Applicant has further found that it is advantageously possible togreatly limit the projection of the closure member head towards theinside of the moulding cavity and therefore, as a result, to remarkablyreduce blend seepages at the inside of the valve duct even when theblend reaches said valve by its tangential movement along the holdingwalls of the moulding cavity, carrying out coupling between the valveseat and the closure member head of said valve in such a manner that, atleast in the closed position, the closure member head keeps at leastpartly embedded in the mould wall.

The Applicant has further found that the above problems can beadvantageously solved even when the closure member head keeps at leastpartly embedded in the mould wall also in the open position of the valveitself.

In a first aspect, the invention relates to a method of manufacturingtyres for vehicle wheels, comprising the steps of:

-   -   a) forming a green tyre;    -   b) closing the green tyre in a vulcanisation mould, the holding        walls of said mould defining an inner surface of a moulding        cavity, said inner surface conforming in shape to the final        conformation to be given to the green tyre;    -   c) pushing the green tyre towards said holding walls of said        mould;    -   d) evacuating a fluid present between an outer surface of said        green tyre and the holding walls of said mould, said evacuating        step comprising the steps of:        -   d1) arranging at least one vent valve in said holding walls,            said vent valve comprising a duct having an inlet end            opening onto said inner surface of said moulding cavity and            a closure member having a closing head which is axially            movable relative to said duct; and        -   d2) transferring said fluid from the moulding cavity to the            duct through a connecting passage confined between an inner            wall of said duct and the closing head of the closure member            set to an open position;    -   e) bringing the green tyre into contact with the closing head of        the closure member to push the latter into the duct so as to        achieve the closed condition of said at least one vent valve and        to mechanically engage the green tyre with at least one grip        element set on said closing head;    -   f) opening the vulcanisation mould in such a manner as to        separate the moulded tyre from the inner surface of said        moulding cavity and to exert a pulling action on said grip        element by the tyre to bring the closure member back to the open        position.

In a further aspect the invention relates to an apparatus formanufacturing tyres for vehicle wheels, comprising:

-   -   devices designed to form a green tyre;    -   at least one vulcanisation mould, the holding walls of said        mould defining an inner surface of a moulding cavity, said inner        surface conforming in shape to the final conformation to be        given to the green tyre, and    -   said vulcanisation mould comprising a plurality of vent valves        for said moulding cavity, at least one of said valves        comprising:        -   a duct having an inlet end opening onto said inner surface            of said moulding cavity, and        -   a closure member having a closing head that is axially            movable relative to the duct between:    -   a closed position, at which said closing head forms a contact        seat with an inner wall of said duct, said contact seat being        defined in said inlet end, and    -   an open position, at which said closing head and inner wall of        said duct confine a passage for connection with said moulding        cavity,        said closing head having at least one grip element to be        mechanically engaged with the green tyre pressed against the        inner surface of the moulding cavity in order to bring the        closure member back to the open position when the tyre is        separated from said inner surface.

Further features and advantages will become more apparent from thedetailed description of a preferred but not exclusive embodiment of amethod and an apparatus for manufacturing tyres in accordance with thepresent invention.

This description will be set out hereinafter with reference to theaccompanying drawings given by way of non-limiting example, in which:

FIG. 1 diagrammatically shows in diametrical section, one half of avulcanisation mould being part of an apparatus for manufacturing tyresfor vehicle wheels in accordance with the present invention;

FIG. 2 is a partial section showing a vent valve according to a firstembodiment;

FIG. 3 shows an enlarged portion of the valve in FIG. 2;

FIG. 4 shows an enlarged portion of a vent valve made in accordance witha second embodiment; and

FIG. 5 shows an enlarged portion of a vent valve made in accordance witha further embodiment.

With reference to FIG. 1, a vulcanisation mould belonging to anapparatus for manufacturing tyres for vehicle wheels in accordance withthe present invention has been generally denoted at 1.

This apparatus generally comprises devices adapted to form a green tyre2 and devices capable of transferring the green tyre 2 into a mouldingcavity 3 of the vulcanisation mould 1. Said tyre-forming andtransferring devices are neither shown nor further described as they canbe made in any convenient manner.

As shown in FIG. 1, the vulcanisation mould 1 has a pair of axiallyopposite cheeks 4 and a plurality of circumferential sectors 5 that,when the mould is closed, define the holding walls 6 of the mouldingcavity 3. The holding walls 6 delimit an inner surface 7 of the mouldingcavity 3 the shape of which matches the final conformation to be givento the tyre. The green tyre 2, once closed in mould 1, is pressedagainst the holding walls 6 by a suitable device 8. Subsequently, orsimultaneously with the pressing step, heat is administered to the greentyre 2 that is pressed against the holding walls 6.

By effect of pressing, suitable ridges set on sectors 5 and cheeks 4cause formation of a desired pattern on the tread band 2 a of the tyre,as well as of a plurality of distinctive graphic marks on the tyresidewalls. The administered heat causes cross-linking of the elastomericmaterial of which the tyre is made up. When a cycle has been completed,the finished tyre (i.e. moulded and cured) is extracted from thepreviously opened mould 1.

Shown in FIG. 1 by way of example is a pressing device 8 comprising abladder 9 of substantially toroidal conformation having twocircumferential edges carrying respective anchoring tailpieces 9 a to besealingly engaged in mould 1. A duct 10 for feeding steam or otherworking fluid and formed in mould 1 opens into bladder 9 so as to enableexpansion of the latter following admission of steam under pressure andto cause compression of the green tyre 2 against the cheeks 4 andsectors 5. Also operatively associated with mould 1, at the cheeks 4and/or sectors 5, are devices designed to supply heat to the green tyre2 to be cured, which preferably co-operate with the steam introducedinto the expandable bladder 9.

The vulcanisation mould 1 further comprises at least one vent valve 11for the moulding cavity 3 which performs the function of, concurrentlywith the pressing step, evacuating air pockets or pockets of other fluidpossibly used in the vulcanisation process, that are present between thegreen tyre 2 and holding walls 6.

As better shown in FIG. 1, the vulcanisation mould 1 comprises aplurality of vent valves 11, preferably mounted at the regions of mould1 close to the tyre shoulders and crown region.

Referring particularly to FIG. 2, each valve 11 comprises a duct 12having an inlet end 13 opening onto the inner surface 7 of the mouldingcavity 3 and an outlet end 14 opposite to the inlet end 13 and openingoutwards of mould 1, through suitable evacuation chambers 14 a (shown inFIG. 1), for example. For conveying the air out of the moulding cavity3, the evacuation chambers 14 a communicate with the externalenvironment of said cavity by means of channels not shown in FIG. 1.

Duct 12 is preferably defined along a valve body 15 inserted in anopening 16 formed in the holding wall 6. The valve body 15 is mountedflush with the inner surface 7 of the moulding cavity 3 in such a mannerthat an edge 15 a of the valve body 15 close to the inlet end 13 issubstantially coplanar with said inner surface 7.

Valve 11 further comprises a closure member 17 provided with a closinghead 18 axially movable relative to duct 12. The closing head 18 isrigidly connected to a stem 19 which is axially and slidably inserted induct 12. Stem 19 brings the closing head 18 close to a first end 19 apositioned in the vicinity of the inlet end 13 of duct 12. A second end19 b of stem 19, opposite to the first end 19 a and placed close to theoutlet end 14 of duct 12, carries a retainer 20 disposed externally ofthe duct itself. The function of retainer 20 is to prevent the closuremember 17 from slipping off the duct 12 towards the moulding cavity 3.Preferably, the retainer 20 is obtained through squashing of the secondend 19 b of stem 19, so as to form a swelling on said end that issufficient not to allow passage of the latter through the outlet end 14of duct 12.

The closure member 17 is movable between a closed position and an openposition. In the closed position (shown in FIG. 2 in solid line), theclosing head 18 forms a contact seat with an inner wall 21 of duct 12,said seat being defined at the inlet end 13, while retainer 20 keepsspaced apart from the outlet end 14. In the open position (representedin FIG. 2 in chain line), the closing head 18 and the inner wall 21 ofduct 12 are mutually spaced apart and confine a passage 23 forconnection with the moulding cavity 3. In this configuration, theretainer 20 lies in contact with the outlet end 14.

A helical spring 24 is disposed around stem 19 and is interposed betweenthe closing head 18 and an abutment surface 25 formed at the inside ofduct 12, preferably close to the outlet end 14. Spring 24 pushes theclosing head 18 to the open position, while the retainer 20 holds theclosure member 17 in duct 12.

Before the pressing step and during the starting moments of this step,the green tyre 2 does not fully adhere to the holding walls 6 of thevulcanisation mould 1 and keeps at least partly separated therefrom.Under this situation, the vent valves 11 are maintained open by effectof springs 24. Compression of the green tyre during the moulding step(for example by means of the pressing device 8) and consequentapproaching of the green tyre 2 towards the holding walls 6 causesevacuation through the open valves 11, of the air (or other fluid)present between the holding walls 6 of the moulding cavity 3 and theouter surface of the green tyre 2. Said air is transferred from themoulding cavity 3 to duct 12 through the connecting passage 23 andtherefrom into the evacuation chambers 14 a. During the pressing stepthe green tyre 2 is brought into contact with the closing head 18 of thevent valve 11, pushing the closure member 17 into duct 12 and causingclosure of valve 11, against the action of spring 24, so as to obstructthe connecting passage 23.

Advantageously, the closing head 18 and the inlet end 13 of duct 12 aresuch shaped that, in the closed position (identified with a solid linein FIG. 3), the contact seat 22 is spaced apart from the inner surface 7of the moulding cavity 3, so that an embedding recess 26 is definedbetween the inner surface 7 and the closing head 18 of the closuremember 17. In other words, in the closed position, the closing head 18keeps at least partly embedded in the inner surface 7.

To this aim, in the preferred embodiments shown in FIGS. 2-5, theclosing head 18 has a peripheral edge 27 that, when valve 11 is in theclosed position, rests against the wall 21 of duct 12 so as to delimitthe contact seat 22. The contact seat 22 lies in a support plane 28 thatis offset relative to the inner surface 7 of the moulding cavity 3 by anembedding distance “d₁”. Preferably, this embedding distance “d₁” isincluded between 0.15 mm and 0.25 mm. As clearly shown in FIGS. 3, 4 and5, the bottom surface of the embedding recess 26 is made up of theclosing head 18 and a surface portion of the inner wall 21 of duct 12included between the closing head 18 and the inner surface 7 of themoulding cavity 3.

In order to obtain the embedding recess 26 when valve 11 is in theclosed position, the inlet end 13 of duct diverges towards the mouldingcavity 3 and opens onto the inner surface 7 at an opening 29 thetransverse sizes of which are greater than the peripheral edge 27 of theclosing head 18. In the embodiments shown, the peripheral edge 27 andopening 29 of duct 12 are of circular shape (in cross section) and thediameter of the edge 27 is smaller than that of opening 29.

Advantageously, the closing head 18 has a proximal portion 30 designedto engage into the inlet end 13 and on top of which there is a thrustsurface 31 facing the moulding cavity 3.

The diverging inlet end 13 has a first angle α₁ defined by the summitangle of the cone tangent to the inner wall 21 of duct 12. The proximalportion 30 tapers towards stem 19 and has a second angle α₂ defined bythe summit angle of the cone tangent to the proximal portion itself. Thefirst and second angles α₁, α₂ have different widths from each other. Inparticular, at least at the contact seat 22 (i.e. in the closed positionof valve 11), the first angle α₁ is advantageously smaller than thesecond angle α₂ in such a manner that the contact seat 22 issubstantially defined (in the corresponding cross section) by acircumferential line. The mutual contact between the closing head 18 andthe contact seat 22 following a circumferential line enables thepossible occurrence of gluing phenomena to be avoided or at leastgreatly limited, between the closing head and inner wall of the duct,due to formation of important thicknesses of cured blend.

Preferably, at least at the contact seat 22, the first angle α₁ has avalue included between 55° and 65°.

Preferably, at least at the contact seat 22, the second angle α₂ has avalue included between 75° and 85°.

As pointed out in the embodiment shown in FIGS. 2 and 3, the inlet end13 of duct 12 has a frustoconical conformation (in the correspondingcross section). The proximal portion 30 too of the closing head 18 has afrustoconical conformation, the major base of which is delimited by theperipheral edge 27. Preferably, the summit angle α₁ of the cone to whichthe frustum of cone of the inlet end 13 belongs is smaller than thesummit angle α₂ of the cone to which the frustum of cone of the proximalportion 30 belongs. Preferably, the diameter of stem 19 is smaller thanthe diameter of the minor base of the proximal portion 30, and anannular surface on which an end of the helical spring 24 rests isdefined between the stem 19 and said proximal portion 30.

Shown in FIG. 4 is a further embodiment, different from the one seen inFIG. 3. In detail, the alternative embodiment in FIG. 4 differs from thevalve in FIG. 3 because the inlet end 13 diverges according to acurvilinear profile and opens in a flaring shape towards the mouldingcavity 3. In this configuration, at the circumferential contact seat 22,the summit angle α₁ of the cone tangent to duct 12 is smaller than thesummit angle α₂ of the cone to which the frustum of cone of the proximalportion 30 belongs.

Shown in FIG. 5 is a further embodiment different from that shown inFIG. 4. In detail, the alternative embodiment in FIG. 5 shows the inletend 13 with a curvilinear profile as that in FIG. 4 and the closuremember 17 comprises an intermediate portion 33 extending in thecontinuation of the proximal portion 30. The intermediate portion 33tapers towards stem 19 following a third angle α₃ of a value smallerthan the second angle α₂. Preferably, the third angle α₃ has a valueincluded between 55° and 65°. Adoption of a summit angle α₃ of aparticularly reduced value enables displacement of the head 18 from theinner wall 21 of duct 12 to be limited, the lift of the closing head 18of valve 11 being the same, and as a result, enables the closure member17 to be guided within duct 12 in a more careful manner, so thatmisalignments of important amount of the closure member relative to theduct are avoided. In other words, the intermediate portion 33 disposedin the extension of the proximal portion 30 gives the closure member 17such a guide action that said member is not submitted to too markedinclinations and the smooth sliding of the latter in duct 12 isimproved.

Alternatively, according to further variant embodiments not shown, theproximal portion 30 of the closing head 18 and possibly also theintermediate portion 33, if present, can have a tapered configuration ofcurvilinear profile.

In addition, still within the scope of the present invention, it ispossible to provide coupling of a closing head 18 with double taper, asshown in FIG. 5, with an inlet end 13 having a frusto-conicalconformation, as shown in FIG. 3.

Alternatively, coupling of a closing head 18 of a curvilinear profilewith an inlet end 13 of a curvilinear profile can be provided, as shownin FIG. 4.

The thrust surface 31 of the closing head 18 carries a grip element 34 athat, together with the thrust surface itself, identifies a distalportion 34 of the closing head 18. In the embodiments shown in FIGS. 2,3 and 5, the thrust surface 31 is flat.

Alternatively, as shown in FIG. 4, the thrust surface 31 of the closinghead 18 is convex and tapers towards the moulding cavity 3 following afrustoconical shape, for example.

As an alternative, the convex surface 31 of the closing head 18 has theshape of a portion of a spherical cap.

In the frustoconical shape, the thrust surface 31 has a major basecoupled with the major base of the proximal portion 30 of the closinghead 18 and delimited by the same peripheral edge 27 as the latter. Theminor base of the thrust surface 31 carries the grip element 34 a lyingspaced apart from the plane containing the peripheral edge 27 of theclosing head 18 by a predetermined distance “d₂”. Preferably, thisdistance “d₂” is included between 0.1 mm and 0.15 mm. Due to theinclination of the convex surface 31 relative to the inner surface 7 ofthe moulding cavity 3, the side thrust exerted by the blend moving alonga direction tangential to the mould surface (i.e. to the inner surface 7of the moulding cavity 3) advantageously generates a component of forceon the closing head 18 which is directed to duct 12 and therefore tendsto close valve 11.

The grip element 34 a delimits a grip recess 34 b on the distal portion34 which is designed to be engaged by the blend of the green tyre pushedagainst the inner surface 7 of the moulding cavity 3. This grip recess34 b is preferably of radial extension. Advantageously this grip recess34 b is defined by an undercut.

In the preferred embodiment illustrated in the figures, the grip element34 a is defined by a frustoconical body having a minor base inengagement with the thrust surface 31 of the closing head 18 and a majorbase lying spaced apart from the thrust surface 31 by a predetermineddistance “d₅”. The side surface of the frustoconical body 34 a delimits,together with the thrust surface 31 of the closing head 18, an annularundercut defining the grip recess 34 b designed to be engaged by theblend. The frustoconical body 34 a preferably has a summit angle α₄included between 30° and 40°.

When the green tyre 2 is pressed against the inner surface 7 of theholding walls 6 and pushes the closure member 17 causing closure of eachvalve 11, the elastomeric material of which the tyre is made, still in araw and particularly fluid state, seeps into the recess 34 b where it iscured and solidifies, following administration of heat usually carriedout within mould 1. Thus a mechanical engagement is created between tyre2 and the grip element 34 a.

When vulcanisation has been completed and the finished tyre (i.e.moulded and cured) is moved away from the walls of the moulding cavity 3due to opening of mould 1, the mechanical engagement of the elastomericmaterial on the closing head 18 causes a pulling action on the gripelement 34 a dragging the closure member along towards the open positionand therefore facilitating the thrust action exerted by spring 24. Whenthe closure member 17 reaches the end of its stroke, limited by theretainer 20, forced disengagement of the tyre from the closing head 18occurs.

Advantageously, temporary engagement of the blend with the closing head18 of the vent valve 11, obtained by the grip element 34 a,ensures-opening of the closure member 17 even when during theVulcanisation process gluing of the closing head 18 onto the inlet end13 of duct 12 has occurred.

In accordance with that which is illustrated in FIGS. 3 and 5, in theclosed position the distal portion 34 is fully housed in the embeddingrecess 26. The major base of the frustoconical body of the grip element34 a is therefore offset relative to the inner surface 7 of the mouldingcavity 3 towards duct 12.

Alternatively, in the closed position, the distal portion 34 is flushwith the inner surface 7 of the moulding cavity 3.

As an alternative, in the closed position, the distal portion 34projects relative to the inner surface 7, as shown in the embodimentillustrated in FIG. 4.

In both these situations, the embedding recess 26 takes the shape of anannular recess.

In accordance with the present invention, the lift of valve 11 is ofsuch a nature that, in the open position of the closure member 17, theperipheral edge 27 of the closing head 18 projects relative to the innersurface of the moulding cavity 3 by a greatly reduced separationdistance “d₃” as compared with the solutions of the known art.Preferably, this separation distance “d₃” is smaller than 0.15 mm. Inthis position, the peripheral edge 27 of the closing head 18 is offsetrelative to the support plane 28 by a distance “d₄” that is greater thanthe distance between the support plane 28 and the inner surface 7 of themoulding cavity 3, i.e. greater than the embedding distance “d₁”.

Alternatively and in accordance with an embodiment not shown, in theopen position of the closure member 17, the distance “d₄” between theperipheral edge 27 and the support plane 28 does not exceed theembedding distance “d₁”. In this case the peripheral edge 27 keepswithin duct 12 (and therefore in the embedding recess 26) even whenvalve 11 is in the open position.

As above described, coupling between the valve seat and the head of theclosure member of said valve is such obtained that, in the closedposition, the head of the closure member keeps at least partly embeddedin the mould wall. This expedient enables projection of the valve headtowards the inside of the moulding cavity to be greatly limited, thevalve lift being the same, when the valve is in the open position. Inthis way the invention, without impairing venting of the air from themoulding cavity, enables elimination of, or great reduction in, theamount of seepage of the blend reaching the head through a tangentialmovement relative to the surface of the mould. In this manner therefore,both the risk of stoppage of the valve and formation of surface faultson the finished products, such as the above mentioned thread-likeoutgrowths, are minimised.

In addition, the reduced projection of the closing head of the valverelative to the inner surface of the moulding cavity enables the thrustexerted by the blend tangentially reaching the closing head and tendingto move the head away from the valve seat to be eliminated or in anycase to be greatly limited.

Finally, the blend reaching the convex surface of the valve head bymoving tangentially of the inner surface of the moulding cavity producesa thrust component intended to close the valve. This thrust is to beadded to the thrust exerted by the blend approaching the closing head ina direction perpendicular to the inner wall of the moulding cavity andfacilitates timely closure of the vent valve, compensating for thetangential thrust exerted by the blend on the lower tapered portion ofthe closing head.

1-74. (canceled)
 75. An apparatus for manufacturing a tyre for a vehiclewheel, comprising: devices designed to form a green tyre; at least onevulcanisation mould, having holding walls defining an inner surface of amoulding cavity, said inner surface conforming in shape to the finalconformation to be given to the green tyre, and said vulcanisation mouldcomprising a plurality of vent valves for said moulding cavity, at leastone of said valves comprising: a duct having an inlet end opening ontosaid inner surface of said moulding cavity, and a closure member havinga closing head that is axially movable relative to the duct between: aclosed position, at which said closing head forms a contact seat with aninner wall of said duct, said contact seat being defined in said inletend; and an open position, at which said closing head and inner wall ofsaid duct confine a passage for connection with said moulding cavity,said closing head having at least one grip element to be mechanicallyengaged with the green tyre pressed against the inner surface of themoulding cavity in order to bring the closure member back to the openposition when the tyre is separated from said inner surface.
 76. Theapparatus as claimed in claim 75, wherein said closing head is providedwith a distal portion facing the moulding cavity and comprising saidgrip element.
 77. The apparatus as claimed in claim 75, wherein saidgrip element delimits on said closing head, a grip recess designed to beengaged by the green tyre.
 78. The apparatus as claimed in claim 77,wherein said grip recess is defined by an undercut.
 79. The apparatus asclaimed in claim 77, wherein said grip element comprises a frustoconicalbody having a minor base in engagement with said closing head and a sidesurface delimiting said grip recess.
 80. The apparatus as claimed inclaim 79, wherein said frustoconical body has a summit angle of 30° to40°.
 81. The apparatus as claimed in claim 75, wherein said contact seatis spaced apart from said inner surface of the moulding cavity so thatan embedding recess is defined between the inner surface of the mouldingcavity and the closing head of the closure member in the closedposition.
 82. The apparatus as claimed in claim 75, wherein the closinghead has a peripheral edge and wherein, in the closed position, saidperipheral edge forms said contact seat in a support plane that isoffset relative to the inner surface of the moulding cavity.
 83. Theapparatus as claimed in claim 82, wherein said support plane is offsetrelative to the inner surface of the moulding cavity by an embeddingdistance of 0.15 mm to 0.25 mm.
 84. The apparatus as claimed in claim82, wherein in the open position of said closure member, the peripheraledge of the closing head projects relative to the inner surface of themoulding cavity by a separation distance smaller than 0.15 mm.
 85. Theapparatus as claimed in claim 82, wherein in the open position of saidclosure member, the peripheral edge of the closing head is offsetrelative to the support plane by an embedding distance that does notexceed a distance between said support plane and the inner surface ofthe moulding cavity.
 86. The apparatus as claimed in claim 75, whereinthe closing head has a convex surface facing the moulding cavity. 87.The apparatus as claimed in claim 76, wherein in the closed position,said distal portion of the convex surface is flush with the innersurface of the moulding cavity.
 88. The apparatus as claimed in claim76, wherein in the closed position, said distal portion of the convexsurface projects relative to the inner surface of the moulding cavity.89. The apparatus as claimed in claim 76, wherein in the closedposition, said distal portion of the convex surface is housed in theembedding recess.
 90. The apparatus as claimed in claim 86, wherein saidconvex surface has a frustoconical conformation.
 91. The apparatus asclaimed in claim 75, wherein a peripheral edge of the closing head liesin a plane which is offset relative to a distal portion of the convexsurface by a predetermined distance of 0.1 mm to 0.15 mm.
 92. Theapparatus as claimed in claim 75, wherein the contact seat is defined bya circumferential line.
 93. The apparatus as claimed in claim 75,wherein the inlet end of the duct diverges toward the moulding cavityaccording to a first angle.
 94. The apparatus as claimed in claim 93,wherein the closing head has a tapered proximal portion having a secondangle different from the first angle.
 95. The apparatus as claimed inclaim 75, wherein the inlet end diverges following a curvilinearprofile.
 96. The apparatus as claimed in claim 75, wherein the inlet enddiverges following a frustoconical conformation.
 97. The apparatus asclaimed in claim 94, wherein said proximal portion of the closing headtapers according to a frustoconical conformation.
 98. The apparatus asclaimed in claim 94, wherein said proximal portion of the closing headtapers according to a curvilinear profile.
 99. The apparatus as claimedin claim 93, wherein at least close to the contact seat said first angleis 55° to 65°.
 100. The apparatus as claimed in claim 94, wherein atleast close to the contact seat said second angle is 75° to 85°. 101.The apparatus as claimed in claim 94, wherein the closure membercomprises an intermediate portion extending in the continuation of saidproximal portion of the closing head.
 102. The apparatus is claimed inclaim 101, wherein said intermediate portion tapers according to a thirdangle having a smaller value than said second angle.
 103. The apparatusas claimed in claim 102, wherein said third angle is 55° to 65°. 104.The apparatus as claimed in claim 75, wherein the duct is defined alonga valve body mounted in an opening formed in a holding wall of themould.
 105. The apparatus as claimed in claim 75, comprising devices forpressing the tyre against the inner surface of the moulding cavity. 106.The apparatus as claimed in claim 75, comprising devices for separatingthe moulded tyre from the inner surface of the moulding cavity.